Precision resistor



PRECISION RESISTOR Filed July n.,

ATTORNEYS United States Patent O U.S. Cl. 338-231 4 Claims ABSTRACT FTHE DISCLOSURE A stable precision resistor which includes a resistorelement comprising a resistive metal lm, having elastic properties whichobey Hooks law in tension and compression, on a rigid dielectricsubstrate which is mounted inside a hermetically sealed oil containinghousing.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is related tothe commonly assigned applications of Felix Zandman and Branin A. Boyd,Ser. No. 453,098, led May 4, 1965 and of John P. Smith IR., tiledconcurrently herewith.

BACKGROUND OF THE INVENTION The present invention relates to electricalresistors, and especially to resistors of extremely high precision andstability.

An object of the invention is to provide resistors of such extraordinarystability with respect to a wide range of ambient conditions and suchpermanence of characteristics as to be suited for use as resistancestandards and to serve other exacting needs.

Various attempts have been made in the past to construct high precisionresistors of specially prepared wire. Winding the wire on spoolsintroduced problems of shortterm and long-term stability of the spoolsthemselves, as to their mechanical and electrical properties. Inattempts to obviate spool difficulties, the wire has in some cases beenloosely rbunched in a container, rather than being wound on a spool.Special alloys have been used, and attempts have been made to minimizethe strains in the wire. Problems of strength and permanence ofstructure arise with such resistors. Furthermore, reactance problems areencountered at high frequencies, because of inductance of the wire,varying with the positioning thereof, and capacitance between theportions of the wire.

In the copending allowed application of Felix Zandman and Branin A.Boyd, cited above, a resistor element is described which overcomes manyof the disadvantages of the best resistors formed of wire. A dielectricsubstrate, such as a thin glass wafer which may be of the order ofonefourth inch square, is provided with a resistive metallic lm in whicha relatively long resistive path is established. As there described,epoxy coatings may be applied to such a substrate, and may besymmetrically disposed to avoid warping tendencies. By taking intoaccount the `ditferent temperature coetlicients of expansion of theepoxy coatings and the substrate, and appropriately relating thereto thetemperature coefficient of resistivity of the resistive metallic lm, theresulting uncased resistor element may be made to have an overalltemperature coeicient of Aresistivity of the order of three parts permillion per degree centigrade over a wide temperature range such as therange from 25 C. to 125 C. Also described in said application of Zandmanand Boyd is the encapsulation of such a resistor element in a plastic,ceramic, or metal housing, which is impermeable to vapor transmission,wherein the resistor unit itself is surrounded by a sheath of rubber,polyurethane foam or other soft material, t-he 3,474,375 Patented Oct.21, 1969 ICC remainder of the space within the housing being filled witha hard ller such as an epoxy.

The resultant encased resistor is characterized by shortterm andlong-term stability characteristics greatly superior to those of othertypes of resistors. Along with its extremely low temperature coefficientof resistivity, it possesses stability with respect to the effects ofmoisture and pressure on the housing which is high enough for manyexacting requirements. In accordance with an important feature of thepresent invention, a resistor element consisting of a lm on a substrate,which may be constructed as described in said prior application Ser. No.453,098 is so housed as to isolate it completely from the effects ofmoisture and variations of pressure on the case, In order to achievethis goal, the resistor element consisting of the substrate and theresistive metallic lm thereon, which has elastic properties obeyingHooks law in tension and compression, is immersed within a selected oiland housed in a hermetically sealed casing, the arrangement of theresistor element being such as to minimize the transmission of anymechanical forces to the element from the housing.

DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS The invention willnow be described in reference to the appended drawings, wherein FIG. lis an exploded view of one embodiment of the present invention and FIG.2 is a detailed view showing arrangement of an enclosure elementapplicable to the structure of FIG. l; FIG. 3 is a view of thecompletely 'assembled resistor unit lcorresponding to FIGS. 1 and 2;FIG. 4 is a cross-sectional View of an alternative embodiment of theresistor unit arranged to include a plurality of resistor waferelements; FIGS. 5 and 6 are an isometric view and a bottom view,respectively, of a multi-cavity dielectric body included in theapparatus of FIG. 4; FIG. 7 is an isometric view of a third embodimentof the invention, a portion of the case being broken away to expose theinterior to view; and FIG. 8 is an exploded view of the apparatus shownin FIG. 7.

Referring now to FIG. l, a metallic cylinder 11 is arranged to receivetwo end pieces, 13 and 14, each of which consists of a glass disk havinga surrounding metal ring fixed to its periphery by a glass-to-metalseal. Each disk also includes a metal eyelet centrally located therein,and similarly bonded to the glass by a glass-to-metal seal. Thesurrounding metal ring of each end of the disk is arranged to be bondedby solder at its periphery to an end of the metal cylinder 11.Preferably, a minute shoulder (not shown) is formed in each end of themetal cylinder 11 to facilitate the accurate positioning of the end diskwith its surrounding metal ring. The central eyelet of each end disk isalso arranged to project a short distance from the Surface of the glassand is prepared to be bonded by solder to a terminal wire lead of theunit.

As one example of mutually compatible glass and metal materials,borosilicate glass can be used with kovar metal.

The assembly to be housed within the cylinder 11 between the end disks13 and 14 consists of a very small printed circuit board 21 upon whichis supported, by its flexible leads 15 and 20, a resistor element 22comprising ya rigid dielectric substrate having a resistive metallic lm,having elastic properties which obey Hooks law in tension andcompression, atlixed thereon. The arrangement and construction of theresistive element 22, arranged for a relatively long conductive pathbetween the junctions of the flexible leads 15 and 20* is described inthe copending Zandman and Boyd application, Ser. No. 453,098.

Also fixed to the printed circuit board 21 are wires 26 and 27 which arearranged to serve the dual purposes of supporting the printed circuitboard 21, and constituting the ultimate terminal Wires or lead wires ofthe completed resistor.

The enclosure, consisting of the metal cylinder 11 and the end disks 13and 14 peripherally bonded to the cylinder and centrally bonded to thelead wires 26 and 27, respectively, is almost entirely filled with asuitable oil, a suitable material for this purpose having been found tobe Dow Corning #200 Silicone Oil. A very small pocket of gas, such asdry air, is provided in order to accommodate differential expansions orcontractions of the housing and the oil contained therein.

The steps involved in assembling the resistor of FIGS. 1, 2, and 3 willnow be described. Initially, one of the end disks 13 is inserted incylinder 11 and its metal ring is soldered in place, making a sealaround the periphery. The resistor unit 22 is connected to the printedcircuit board 21, the end of flexible lead 1S being soldered to ltheprinted circuit conductor which extends to lead 26, and the end offlexible lead being soldered to the printed circuit conductor leading tothe junction of lead 27. The leads 1S and 20 are bent after being bondedto the respective conductor portions of the printed circuit board 21.Preferably, before inserting the assembly including resistor 22, printedcircuit board 21, and the ultimate resistor terminal leads 26 and 27into the cylinder 11, this cylinder is lined with a thin layer of Tefloninsulating material to insure against any accidental electrical contactbetween either of the units 21 or 22 and the inner cylindrical wall ofthe housing.

The lead wire 26 is inserted through the eyelet of disk 13 as thesub-assembly, including the printed circuit board 21 and the resistor22, is moved to the left and into the interior of the cylinder 11. Whenthe sub-assembly, including parts 21 and 22, is approximately centrallylocated within the cylinder 11, a soldered junction is formed betweenthe central eyelet of disk 13 and the lead 26, completing hermeticsealing of the left end of the resistor unit.

Next, the opposite end disk 14 is slipped over the terminal lead wire 27and moved into position in the end of cylinder 11 opposite the first enddisk 13. The peripheral metal ring of end disk 14 is soldered to thecylinder 11 around its entire periphery by dipping this end of thecylinder (along with the projecting terminal lead 27) into a hot solderbath. During this operation, the solder is prevented from bonding thewire 27 to the eyelet of the right hand disk 14 by the heated airescaping through said eyelet from the interior of the cylinder 11.

The unit as thus far assembled has the same appearance essentially asshown in FIG. 3, the only things lacking at this stage being the almostcomplete filling of oil in the solder junction between terminal leadline 27 and the eyelet of end disk 14. The structure is then placedwithin an evacuation chamber in which is a reservoir of the siliconeoil. In that chamber, the air contained within the structure issubstantially completely exhausted, and by virtue of immersion of theunit in the silicone, restoration of atmospheric pressure causes thesilicone oil to be drawn into the housing substantially filling it.Before making the final solder junction between the terminal lead wire27 and the righthand end disk 14, the temperature of the unit iselevated to approximately 125 C., at which temperature the silicone isexpanded to a greater than normal volume. Thereafter, cooling of theunit to room temperature results in the ingress of a very small amountof air, to provide the desired pocket for expansion and contraction. Theproduct is completed by forming the solder junction between the terminallead wire 27 and the eyelet of end disk 14.

For some ranges of resistance, and for increased heat dissipationcapacity, it is desirable in some instances to use several resistorelements within the housing defined by cylinder 11 and end disks 13 and14. In FIGS. 4, 5, and 6, a modification is illustrated wherein thereare provisions for connecting three separate resistor units, each likethe resistor element 22 in FIG. l, in a series and housing them withinthe oil bath in the enclosure defined by cylinder 11 and end disks 13and 14. For this purpose, a plastic block 41 having a plurality ofcavities 42, 43, and 44, may be provided and so arranged as to beinserted within the metal cylinder 11. The plastic block 41, shown incross-section in FIG. 4 and its oblique and bottom views in FIGS. 5 and6-, respectively, provides for the positioning of a plurality of theresistor elements in respective ones of its cavities, and for theconnection of their respective flexible leads through holes in thebottom of the block 41 to a pattern of printed circuit conductors on thebottom of said block (FIG. 6). As shown in FIG. 6, the pattern ofconductors on the bottom of the block 41 may be so arranged as to causethe plurality of resistor elements to be connected in series with eachother between the left hand terminal lead 26 and the right hand terminallead 27.

The internal dimensions of the cavities 42, 43 and 44 are substantiallygreater than the respective dimensions of the resistor element, such asthe resistor element 22 of FIG. 4. Accordingly, when the resistorelement is properly positioned and installed within its cavity of theblock 41, it is surrounded entirely by oil, and is free from anymechanical forces exerted by the block 41 or any portion of theenclosures cylindrical or end parts.

In FIGS. 7 and 8 is shown an alternative construction of a resistor inaccordance with this invention. Referring now to FIGS. 7 and 8, asingle-cavity of a molded plastic container 51 is arranged toaccommodate a single film-onsubstrate wafer resistor 22, the cavityagain being substantially larger in all its dimensions than thedimensions of the resistance wafer element. The housing for the resistorof FIGS. 7 and 8 comprises a relatively narrow metal cup 52 open only atits lower end, and an end closure unit 53 comprising a glass body 54having two eyelets 56 and 57 bonded thereto by glass-to-metal seals,along with a peripheral metal bond 58 also joined to the glass in aglass-to-metal seal.

In this form of a resistor, the terminal lead wires 26' and 27 are bothextended downward from the bottom of the resistor unit parallel to eachother.

The single-cavity body 51, unlike the molded plastic unit 41 on themulti-cavity resistor embodiment, may omit any printed wiring patternand rely instead upon mere passages or bottom holes through which theresistor terminal leads are to extend.

The assembly of FIGS. 7 and 8 is made up by rst bonding the ends of therelatively short flexible leads 15 and 20 from the iilm-on-substrateresistor element 22 to the upper ends of the terminal lead wires 26' and27. These terminal lead wires are -then passed downward through thecavity of the molded body 51, and extended through the holes in thebottom of said body. The terminal lead wires 26 :and 27' are then passedthrough the metal eyelets 56 and 57 which are sealed to the glass 54 ofthe end closure unit 53.

A very small layer of Teflon, or other suitable insulating material,(not shown) is inserted within the metal can 52 and made to lie againstthe closed end thereof. The molded body 51 having the resist-o-r waferelement 22 enclosed therein is -moved up into the interior of the can52, and the end closure unit 53 is next brought into a position on thelower end of the can 52, where it is ready to be soldered in place.

The unit is Ithen soldered by dip soldering, to form a continuous andcomplete bond 4between the lower end of the can 52 and the metal ring 58bonded to the periphery of the lower end closure unit 53 and forming apart thereof'. In this dip-soldering process, one of the eyelets in lthelower end of the closure unit will be solder-bonded to the terminal leadwire passing therethrough, but the other will be kept open by theemerging air due to the rising temperature within the can 52. Again, theprocess of substantial evacuation is followed by filling with thesilicone filler at the elevated temperature, after which the siliconecontracts leaving a small void sufficient to allow for the differentialexpansion and contraction of the case and the silicone filling. Thefinal step, in this embodiment, as in the previous embodiment, is thesoldering of the open eyelet to the terminal wire lead passingtherethrough.

The resultant resistor in any of the embodiments illustrated in thedrawings is better capable of standing shock and vibration than any ofthe loosely wound wire precision resistors, and is at least as good asthe spoolwound precision resistors. In contrast to both such types ofwire resistors, it has a minimum of reactive effect, its inductancebeing typically as low as or lower than 1/10 microhenry, and itsdistributed capacitance being typically as low as or lower than 1/2micromicrofarad. By virtue of such extremely low reactance factors, theresistor in accordance with the present invention remains at asubstantially unity power factor at frequencies far greater than at thefrequencies up to which the resistors constructed of wire may be used,with -or without attempts at inductance cancellation arrangements.

Whereas, the prior form of wafer resistor, embedded in a soft cushionwhich in turn is within an epoxy filling in a case having epoxy end sealor seals, has some susceptibility to moisture effect and to variationsof pressure ent invention is rendered independent of surrounding have asmuch as 50 p.p.m. change of resistance due to moisture even steam underpressure) and independent of unit is used, the resistor unit inaccordance with the presof the medium (frequently the atmosphere) inwhich the encased wafer resistor, with its resistive film path, couldpressure of 'the surrounding medium. The former type of change ofpressure, and as much as 400 p.p.m. change of resistance with moistureunder extremely adverse condi- `ti-ons. In contrast, the resistor inaccordance with the present invention demonstrates insufficient changeof resistance in response `to either of these effect-s to be measurablewith extremely sensitive measuring equipment.

Also, while the change of resistance value with aging (for example,aging on the shelf) is less for the previously described epoxy-sealedresistor from that which applies to an unencased wafer resistor unit,the resistor constructed in accordance with the present inventiondemonstrates such extraordinary freedom from a drift with aging as toshow no measurable a-mount on equipment arranged for checking to theresolution of one p.p.m. In this respect, also, the precision resistorin accordance with the present invention not only is at least an orderof magnitude'better than the encased wafer resistor heretoforedeveloped, but also is an order of magnitude better than the precisionresistors which have been made using specially prepared wire, eitherloosely inserted or spooled.

The resistor, in accordance with the present invention, has theproperties of a resistor standard. Inasmuch as 'the resistor isextremely compact, it is possible to prepare -a versatile, multivalueresistor standard by the use of switching means, as set forth in theabove-identified ap plication of John P. Smith, Jr.

It will be understood that various modifications can be made withoutdeparting from the invention.

What is claimed is:

1. An ultra stable precision resistor comprising:

a housing impermeable to vapor transmission;

at least two resistor terminal wires extending outward from saidhousing;

at least one of said wires being insulated from said housing;

a resistor element within said housing including a rigid dielectricsubstrate having la resistive metal film thereon and a pair of flexibleleads connected to said resistive film and extending from saidsubstrate;

said resistive film having elastic properties which obey Hooks law intension and compression;

means including said flexible leads for establishing a resistive pathbetween said two resistor terminal wires through said resistive film;

means for hermetically sealing said housing at at lea-St one end thereofand affording insulated support for at least one of said resistorterminal wires;

said means including a glass portion sealed by glass-tometal seals atits periphery and at the point of support of the resistor terminal wire;

and an oil medium sealed within said housing surrounding said resistorelement and filling the majority of the enclosed volume.

2. An ultra stable precision resistor as defined in claim 1, whereinsaid housing is cylindrical and said resistor terminal wires extendaxially in opposite directions from said housing, said means forhermetically sealing said housing including a pair of symmetricallyarranged end closure units each comprising a glass disk with peripheraland central metal-to-glass seals.

3. An ultra stable resistor as defined in claim 1, wherein said housingis a cup and said -means for hermetically sealing said housing comprisesa glass body through which both said resistor terminal wires extend.

4. A precision resistor comprising:

-a metal housing;

two resistor terminals including at least one resistor terminalconductor extending outward from said housing and insulated therefrom;

a resistor element within said housing including a rigid dielectricsubstrate having a resistive metal film thereon and a pair of connectingleads extending from said substrate for establishing a circuit paththrough said film;

said resistive film having elastic properties which obey Hooks law intension and compression;

means providing conductive paths between said connecting leads and Saidtwo resistor terminals;

means for hermetically sealing an end of said housing and affording aninsulated support for said resistor terminal conductor, said last namedmeans including a glass portion sealed by metal-to-glass seals both atits periphery and at the point of support of the resistor terminalconductor;

and an oil medium sealed within said housing surrounding said resistorelement and incompletely filling the enclosed volume.

References Cited FOREIGN PATENTS 1,111,248 4/ 1968 Great Britain.

ELLIOT A. GOLDBERG, Primary Examiner U.S. Cl. X.R.

